Your eyes compute gradients, as part of the shitton of visual processing your brain does to get an estimate of where the local and global minima are.

It is not perfect though, see the many optical illusions.

But we follow gradients all the time, consciously or not. You know you are at the bottom of the hole when all the paths go up for instance.

It has been suggested [citation needed] that the optical illusions of movement caused by gradients are there to to compensate for the time it takes to process the visual input. This should let you have an understanding of what is going on in the world around you right now, based on what happened on your retinas a few milliseconds ago.

it's not a bug - it's a feature :D